1. Field of the Invention
This invention relates to a process for the manufacture of mixtures of diphenylmethane diisocyanate isomers, particularly of mixtures consisting of 2,4'- and 4,4'-diphenylmethane diisocyanates, as well as pure MDI, 4,4'-diphenylmethane diisocyanate, containing a small amount of uretdiones and hydrolyzable chlorine by distillation of crude MDI. The crude MDI is a mixture of diphenylmethane diisocyanates and polyphenylene polymethylene polyisocyanates, in which the distillable diphenylmethane diisocyanate isomer mixtures and/or the 4,4'-diphenylmethane diisocyanate are/is condensed in two stages, possibly in the presence of an inert gas.
2. Description of the Prior Art
In addition to crude MDI, 2,4'-diphenylmethane diisocyanate, and particularly pure MDI, 4,4'-diphenylmethane diisocyanate, are important raw materials for the manufacture of polyurethane plastics.
A number of methods are known which describe the simultaneous manufacture of pure MDI and polyphenylene polymethylene polyisocyanates. German application No. 15 93 638, German application No. 16 43 093, German application No. 19 23 214, and German Published application No. 24 04 170 relate to distillation processes for the separation of crude MDI.
According to German Published application No. 21 05 193 and German Published application No. 24 25 658, the diphenylmethane diisocyanate isomers are initially distilled from the crude MDI and are then separated into pure MDI and a mixture of 2,4'- and 4,4'-diphenylmethane diisocyanate by fractional crystallization.
A drawback of the described processes is that the resultant isocyanates contain varying quantities of uretdiones and chlorine compounds as byproducts.
Uretdiones are produced in a temperature dependent equilibrium by containing dimerization of 4,4'-diphenylmethane diisocyanate at the NCO groups. Uretdiones form insoluble crystals in pure MDI plugging lines, equipment and machinery during processing.
Organic chlorine compounds are generally detrimental if they are converted into compounds with easily hydrolyzable chlorine at elevated temperatures. Hydrolyzable chlorine compounds, particularly when they occur in varying concentrations, interfere with the reaction of isocyanates with polyols resulting in polyurethanes since the rate of reaction is influenced by the chlorine compounds. In addition to this, they cause a more rapid yellow discoloration of the initially water clear and colorless isocyanates. It is difficult to analytically differentiate among the individual chlorine compounds. The following are mentioned as possible compounds: phosgene, N,N-dimethylaniline hydrochloride, N-chloroformylaniline, N-methyl-N-chloroformylaniline, as well as compounds having the formulas: ##STR1##
In order to reduce the concentration of hydrolyzable chlorine, it has been suggested that the organic isocyanates, possibly in the presence of metal compounds, are heated to increased temperatures (U.S. Pat. Nos. 3,155,699, 3,264,336 and German application No. 12 70 036) and that they are subsequently distilled or that the chlorine compounds are separated by a commercially expensive fractional crystallization (German Published application No. 19 38 384).
In order to separate crude MDI and to manufacture diphenylmethane diisocyanate isomers having an adjusted chlorine content, the materials are subjected to a 4 or 5 fold fractional distillation in accordance with German application No. 26 31 168. According to data in this publication (FIG. 3), the diphenylmethane diisocyanate isomers are removed from the crude MDI by distillation in the first column. The diphenylmethane diisocyanate isomers are then separated in a second column from small quantities of distilled over 3-nuclei oligomers and hard to volatilize compounds containing hydrolyzable chlorine. In a third column, the lower-boiling chlorine compounds, which are distilled off as head fractions, are separated from the diphenylmethane diisocyanate isomers. The diphenylmethane diisocyanate isomers consisting of 2,4' and 4,4'-diphenylmethane diisocyanates are subjected to another distillation for the purpose of separating the isomers whereby the lower-boiling 2,4'-diphenylmethane diisocyanate is distilled overhead. In a last distillation stage, the remaining 4,4'-diphenylmethane diisocyanate must then be separated from the polymer components produced during the distillation. A drawback of this method is that the thermally sensitive isocyanates polymerize during the multiple distillation, thus forming a relatively large polymeric residue.